Accumulation of Naringin and Limonin and Genes Expression of their Corresponding Enzymes during Fruit Maturation of Three Citrus Genotypes

Naringin and limonin are important antioxidant compounds in Citrus species. Due to the high biological effects of these compounds, in the present study, the variation of naringin and limonin content during fruit development was investigated at two fruit tissues of pulp and albedo in three Citrus genotypes (grapefruit, orange and mandarin). Also, the gene expression of involved enzymes in naringin and limonin metabolism was assayed. In all studied Citrus genotypes, the naringin content in albedo tissue was higher than the pulp and grapefruits showed the highest naringin content (940.26 mg per 100 g DW) followed by orange (791.17 mg per 100 g DW) and mandarin (602.10 mg per 100 g DW). The naringin content of studied genotypes was decreased during the fruit development and the highest decrement was observed at ripening stages. The decreasing of naringin content was attributed to the changes in genes expression of main involved enzymes in naringin biosynthesis pathways including chalcone-flavanone isomerase and naringenin glycoside biosynthesis (1.2 RhaT) during studied harvesting times. In both tissues of mandarin and orange fruits, the limonin content was increased up to 80 days after full bloom then decreased. In grapefruit, due to the late-ripening of the fruit, the peak of limonin increment was occurred later, which can be attributed to the long growth process of grapefruits compared to orange and mandarin. The reduction of limonin content with fruit ripening was in accordance with the increased gene expression of limonoid UDP-glucosyl transferase that converts limonin to tasteless compounds.